| Summary: | Komatiites contain some of the world's highest-grade nickel-sulphide deposits. Accordingly, the development of reliable prospectivity indicators to guide exploration towards mineralised komatiite units has been a long-standing goal. Specifically, the development of effective resistate indicator minerals, analogous for example to the use of garnet in diamond exploration, would be pivotal in the exploration for mineralised komatiites. We utilised laser ablation ICP-MS to measure the ruthenium (Ru) concentrations of more than 390 chromite grains from barren and mineralised komatiites from the Yilgarn Craton of Western Australia and the Finnish part of the Fennoscandian Shield. The results show that chromites from barren komatiite systems yield Ru concentrations mostly between 150 and 600 ppb. Conversely, chromites from mineralised units have distinctly lower Ru contents (<150 ppb). These results can be interpreted in terms of the much higher partition coefficient for Ru into sulphide liquid compared with that into chromite, resulting in much lower Ru concentrations in chromite where both chromite and sulphide liquid are present and competing for Ru. Since chromites from mineralised komatiites have distinctly lower Ru contents than chromites from barren environments, we suggest that Ru content in chromite can be used as an effective prospectivity indicator. Due to the high immobility of Ru during postmagmatic processes and the relatively strong resistance of chromite grains to destructive metamorphism, alteration and lateritic weathering, detrital chromite from heavy mineral separates can potentially be used to evaluate terranes for nickel-sulphide mineralisation.
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